If you still see the amount in your balance then you need to go back to the purchase screen and actually buy the add-on. But check on the plan details page if you only bought talk and/or text. Data will show up under my data & add-ons.
Adding - so you had bought it earlier and it worked briefly but now not? Try to manually connect to T-Mobile. Also try to set the phone to 2G-only if that's an option on your phone to see if talk works.
If you followed instruction to the letter, your add-on should be on your account.
Purchasing an add-on after logging in into your PM account is a two-step process. You need to load funds first and then purchase add on.
Purchased Add-ons should show up on Plan Details page. Bonus add-on should be on Overview/Main page.
All countable features will show up on the overview page. Unlimited features show up in Plan Details. ie. roaming talk and text.